def find_max(numbers):
    if len(numbers) == 0:
        return None

    max_value = numbers[0]  # ← 断点 1

    for num in numbers[1:]:  # ← 断点 2
        if num > max_value:
            max_value = num

    return max_value  # ← 断点 3

test_list = [3, 7, 2, 9, 1, 5]
result = find_max(test_list)
print(f"最大值: {result}")